Conventionally, (brushless motors), which include induction motors using inverters, semiconductor motors and colorless motors having a small inductance, are employed as high speed motors. However, a motor of this kind is large in size and high in cost because it generally comprises a direct current power source for rectifying an output from an alternating current power source. The above defect becomes significant, particularly, in the case of a motor including an inverter. Further, the maximum rotation speed of the conventional electric motor varying from approximately 3000 to 6000 rpm decreases below the above rotation speed when the motor is arranged to generate a high power output. When an increased voltage is applied to the electric motor so as to increase the rotation speed, the efficiency of the electric motor is lowered to the extent that it becomes particularly useless. Heretofore, no electric circuit is provided, which is adapted to be mounted on the motor for constant torque driving, constant speed driving or servo control and is stably operable at high speeds. In particular, efficiently operable one is not developed.